Treated water that's too pure lets arsenic sneak in

Orange County is tackling California’s water shortage by recycling wastewater into tap water. (The appearance in this photograph of Lake Mission Viejo in Mission Viejo, Calif., is entirely coincidental and in no way intended to imply that the lake is or has ever been exposed to arsenic contamination as a result of the phenomenon detailed in the research covered in this article.) Credit: WPPilot, CC BY-SA 3.0.

With California’s water resources dwindling to alarmingly low levels, the Orange County Water District (OCWD) has pioneered a high-tech approach for recycling wastewater into potable tap water instead of discharging it into the ocean. The purification process is so thorough, however, that it might actually make the water too clean: In a new study, researchers have found that the ultra-purified water is vulnerable to contamination by naturally occurring arsenic in underground storage aquifers.

“Normally, a wastewater treatment plant takes the water that runs down our drains and diminishes the nutrient load and screens out the contaminants,” says Scott Fendorf, a biogeochemist at Stanford University and an author of the new study, published in Environmental Science & Technology. Then, the facility discharges the treated water into the ocean or a river, “which is a huge waste of the resource.”

Since 2008, Orange County has taken a different approach by recycling its wastewater with a rigorous treatment program that includes microfiltration, reverse osmosis, and a final cleansing with ultraviolet light and hydrogen peroxide. “This purification process is arguably one of the most advanced water treatment systems in the world. It produces water that’s as pure as you can get it,” Fendorf says.

After purification, the water is piped 20 kilometers away where it’s allowed to seep into aquifers and mix with groundwater. There it has a residence time of at least six months before it is pumped out again and released to the county’s 2.4 million residents for use. Close monitoring by the OCWD of the water returned for storage revealed that the ultra-purified water was picking up trace amounts of arsenic from sediments above the aquifers. Natural groundwater in the aquifers, on the other hand, which was distinguished from the treated water using organic tracers and is naturally arsenic-free, was not attracting the contaminant.

Arsenic levels in the stored water were never dangerously high during monitoring — only occasionally rising just above the EPA’s limit for public water systems of 10 micrograms per liter of water — and the spikes tended to be transient, naturally falling to acceptable background levels before the water was extracted from the system.

“At no point was the groundwater delivered for public consumption in the area unsafe, but the OCWD was considering expanding its recharge of purified recycled water, so we thought it was prudent to get a better understanding of what was going on,” Jason Dadakis, OCWD’s director of health and regulatory affairs and an author of the study, said in a statement released by Stanford.

Looking for solutions to the problem, Fendorf’s team found that two main factors that contribute to groundwater arsenic contamination elsewhere — anaerobic conditions brought on by microbial activity and high pH levels — did not appear to be causing the OCWD’s arsenic issues. However, they did identify the source of the arsenic: a thin band of clay above the aquifer. And in a series of experiments, they determined that the problem was that the treated water was too pure — specifically, it was lacking in calcium and magnesium. This deficiency caused calcium and magnesium in the naturally occurring clay to migrate into the water so it was in equilibrium with its surroundings.

“When that happens, some of the arsenic ions go with the calcium and magnesium ions, contaminating the water supply,” Fendorf says. The natural, untreated water in the aquifer didn’t trigger the same mechanism because it contained ample calcium and magnesium to maintain equilibrium. “This is a new trigger for arsenic contamination that hasn’t been appreciated before,” Fendorf said in the statement. The OCWD now mitigates the problem by adding calcium, in the form of lime, to their storage aquifers, which helps maintain equilibrium in treated water and keeps the natural arsenic in the clay layer.

The lessons learned in Orange County could have wider applications, especially as more communities turn to subsurface water storage facilities, says Chelsea Neil, a chemical engineer at Washington University in St. Louis who was not involved in the new study.

In addition, the study “highlights the untapped capacity for using clay minerals to absorb arsenic” from water, Neil says, the opposite of the pathway observed in Orange County where arsenic was leaching from clay. Particularly in recharge basins where water is passed through a screening layer before it enters shallow aquifers, clays could be used to help remove arsenic.

Clay minerals are not as effective as other materials such as iron or aluminum hydroxides for this sort of screening, Neil says, “but they have the benefit of being cheap and naturally abundant, and they usually don’t have any adverse effects on the environment.”

“Arsenic in drinking water supplies is a big issue in many different places in the world,” Neil says. “Studies like this may inspire some new ways to mitigate the problem.”